The present invention relates to reproducing a digitized image, particularly but not exclusively by means of a printing technique that makes use of an ink transfer member on which the image has been converted into a half-tone image.
In such an image, also known as a screened image, the tones of the pixels in the digitized image are converted into selected ratios of inked area to non-inked area.
In the printing method using a screen with amplitude modulation, the tone values of the pixels in the digitized image are converted into selected ratios of inked area to non-inked area by modulating the size of each point in the screen as a function of the value of the tone to be reproduced.
FIGS. 1 to 16 show this principle, it being assumed for reasons of clarity of explanation that the tone of each pixel in the digitized image is encoded on 26 values only.
On this assumption, the area allocated in the half-tone image for reproducing each pixel of the digitized image, also referred to as a xe2x80x9cbase cellxe2x80x9d, is subdivided into 25 identical unit zones as shown in FIG. 1, and the same applies to the ink transfer member used for printing the medium, and to the medium printed in this way.
As a function of the tone to be reproduced, the screen dot fills a larger or smaller number of unit zones, with filling beginning in the center when an amplitude modulated screen is used, as shown in FIGS. 2 to 16.
In these figures, each unit zone is given a number that corresponds to the tone value above which that zone is to be covered in ink.
FIGS. 2 to 6 show respectively the unit zones which are inked to reproduce tone values going from 1 to 5, FIGS. 7 to 11 respectively represent those which are inked to represent tone values going from 11 to 15, and FIGS. 12 to 16 show the zones which are inked respectively for representing tone values going from 21 to 25.
FIG. 17 shows the tone values of the pixels in a digitized image that is to be reproduced, corresponding to toning varying from high light level near the bottom (i.e. small tone values) towards low light levels near the top (i.e. higher tone values).
Assuming that the printing technique used for printing the image shown in FIG. 17 has no physical limitations, then the distribution of inked areas both on the ink transfer member and on the printed medium will be as shown in FIG. 18.
Unfortunately, in practice, it turns out that known printing techniques do not enable dark and/or light tones to be reproduced faithfully, particularly because of physical limitations.
In particular, if the printing technique used is flexographic printing, i.e. if it implements an ink transfer member which is etched so as to have relief at those locations where ink is to be transferred onto the medium that is to be printed, it is observed that isolated portions of relief of small size that are intended to reproduce pale tones cannot be achieved or cannot be retained durably because they are too fragile.
The same problem arises with dry offset printing.
To illustrate the above, it can be assumed that portions of relief of a size corresponding to less than four unit zones in FIG. 18 cannot be made physically on the ink transfer member, such that the image as actually printed on the medium will be as shown in FIG. 19.
To illustrate the consequences on the visual appearance of the printed image of failure to reproduce pale tones, it is appropriate to compare FIGS. 20 and 21.
FIG. 20 shows an image that reproduces all of the tones, from the palest values to the darkest values.
In FIG. 21, the same image is shown, but with no toning when the value of the toning is less than a given threshold, which threshold depends in practice on physical limitations specific to the printing technique used.
Problems also arise in reproducing dark tones, particularly when the printing technique used is flexographic printing or dry offset printing, since although the strength of the portions in relief formed on the ink transfer member is no longer an issue, there is then a tendency for isolated unit zones that are not inked to become plugged because of the phenomenon of ink accretion during printing.
This means that it is not possible to reproduce dark tones on the printed medium.
Furthermore, in the printing method that uses a so-called xe2x80x9crandomxe2x80x9d screen, conversion of each tone value into a selected ratio of inked area to non-inked area in the base cell is performed by filling unit zones, not starting from the center, but in a random or pseudo-random manner so that the inked unit zones are distributed over the entire area of the base cell, as shown in FIGS. 22 to 37.
In FIG. 22, the unit zones are numbered in the order in which they are successively covered with ink as the value of the tone to be reproduced increases.
Thus, FIGS. 23 to 27 show the unit zones that are inked to represent tone values 1 to 5, respectively, FIGS. 28 to 32 show the zones which are inked to present tone values 10 to 14, respectively, and FIGS. 33 to 37 represent the zones which are inked to represent tone values of 21 to 25, respectively.
When the image to be reproduced has tonings with tone values that increase when going upwards, as shown in FIG. 17, then, for a printing technique that suffers from no physical limitation, the inked area distribution shown in FIG. 38 would be obtained on the inking transfer member and on the printed medium.
However, the reproduction of pale tones suffers from the physical limitations of the printing technique used, such that in fact it is not possible to reproduce in satisfactory manner on the printed medium all of the tones in the digitized image.
Proposals have been made to remedy the problem of reproducing pale tones by printing the zones of the image concerned by this problem with a screen at half the pitch so as to benefit from bigger screen points.
Nevertheless, the transition between zones using screens of different pitches can clearly be seen on the image that is obtained in this way, and that is undesirable.
One such image is shown in FIG. 39.
In general, whatever the printing technique used, e.g. wet offset, dry offset, gravure, silk screen, rubber stamping, flexographic printing, or indeed ink jet printing, the reproduction of tones that are pale and/or dark gives rise to problems, whether for reasons of mechanical strength concerning portions in relief or plugging while printing is taking place, or for other reasons such as the diffraction of light by the photographic film used for photoengraving or for photosensitizing the ink transfer member, or indeed because of phenomena associated with ink accretion, drying, surface tension, etc.
European patent application EP-A-0 740 457 discloses a method in which a changeover is made from amplitude modulation to frequency modulation for pale tones. That method makes it possible to avoid problems associated with reproducing points in pale areas that are encountered with amplitude modulation. Nevertheless, that method does not always provide results that are visually satisfactory since the eye tends to perceive the zone of the image which is screened using frequency modulation as suffering from a defect since the spacing between the points appears irregular compared with the remainder of the image which is screened using amplitude modulation.
The present invention seeks to improve reproduction of pale and/or dark tones, whatever the printing technique used.
In particular, the invention provides a conversion method for generating a half-tone image from a digitized image comprising a set of pixels each having a tone value, with each pixel of the digitized image being associated with a base cell of the half-tone image being subdivided into as many unit zones as there are non-zero tone values, the method comprising the steps consisting in:
analyzing the digitized image in terms of analysis windows each constituted by grouping together a predetermined number of pixels; and
at least when the tone value of at least one of the pixels in an analysis window falls within a first predetermined set of tone values, converting the tone value of each of the pixels of the analysis window into respective selected ratios of inked area to non-inked area in the various base cells corresponding respectively to said pixels, said base cells together forming a supercell, said conversion being performed by complying with a predetermined distribution and filling order of unit zones that is a function of the tone value to be converted, said distribution and said filling order taking account of the printing technique used for reproducing the half-tone image on a medium, the resulting image having two zones with different screen pitches without any sharp transition therebetween.
In a particular implementation, conversion to respective selected ratios of inked area to non-inked area in the various base cells of the supercell is performed by complying with a predetermined distribution and filling order for unit zones that is a function of the tone value to be converted, and doing so regardless of the individual tone values of the pixels in the analysis window.
Still in a particular implementation, when all of the tone values of the pixels in the analysis window fall within a second predetermined set of tone values, the unit zones of all of the base cells of the supercell are filled in the same manner.
Advantageously, the predetermined distribution and filling order serve to group together filled unit zones within the supercell when the tone value of at least one of the pixels of the analysis window is below a given threshold.
Preferably, the filled unit zones are grouped together in slabs of n unit zones within each base cell belonging to the supercell, n being a multiple of the number of base cells constituting the supercell.
Advantageously, the predetermined distribution and filling order serve to group together empty unit zones within the supercell when the tone value of at least one of the pixels of the analysis window is greater than a given threshold.
Preferably, the empty unit cells are grouped together in slabs of n unit zones within each base cell belonging to the supercell where n is a multiple of the number of base cells constituting the supercell.
Each slab may be positioned in the center of the base cell in which it is to be found.
In a variant, each slab may be positioned off-center within the base cell in which it is to be found.
The invention also provides a method of reproducing a digitized image by a printing technique such as wet offset, dry offset, gravure, silk screen printing, rubber stamping, or flexographic printing, implementing an ink transfer member on which the tones of the pixels of the image to be reproduced are converted into selected ratios of inked areas to non-inked areas by the above-specified conversion method.
By means of the method, it is possible to reproduce various tones that are pale and/or dark more faithfully in spite of the limitations of the printing technique used.
In the invention, dots are built up within each supercell by using predetermined geometry, and not in random manner. In other words, dots are constructed for pale and/or dark tones without using frequency modulation, but by using predetermined filling and distribution orders enabling the unit zones inked in pale tones and/or the empty zones in dark tones to be grouped together, thereby obtaining two zones of different screen pitch without any sudden transition between them.
The invention makes it possible in particular to reproduce variations in toning within pale tones because the unit zones that are inked are no longer isolated.
Thus, when the printing technique is dry offset printing or flexographic printing that implements portions in relief on the ink transfer member, the fragility of said portions in relief is reduced because they are of larger size since they are organized to reproduce slabs made up of a plurality of unit zones, and not unit zones that are isolated.
Similarly, regardless of whether the printing technique is gravure and implements recesses in the ink transfer member, silk screen printing implementing openings in the ink transfer member, or indeed wet offset printing implementing repulsion between oil and water on the ink transfer member, the invention makes it possible, by grouping together the inked unit zones that are to reproduce pale tones or the uninked zones that are to reproduce dark tones, to avoid being confronted with the above-mentioned problems associated with said unit zones being isolated.
In addition, with photoengraving or photo-sensitization of the ink transfer member, light diffraction phenomena are reduced because the unit zones that have been grouped together in this way diffract light to a lesser extent than do isolated unit zones.
The invention also provides a method of reproducing a digitized image by an ink jet printing technique, wherein a half-tone is printed as obtained by the above-specified conversion method.
Under these circumstances, a half-tone image is reproduced more faithfully when the toning is dark, since the distortions due to the phenomenon of accretion are reduced.
The invention also provides an ink transfer member comprising a zone for reproducing various pale tones, wherein said zone has a plurality of inked areas of identical dimensions and non-inked areas of dimensions greater than or equal to those of a base cell.
The invention also provides an ink transfer member comprising a zone for reproducing various dark tones, wherein said zone has a plurality of non-inked areas of identical dimensions and inked areas of dimensions that are greater than or equal to those of a base cell.
The invention also provides a printed medium comprising a zone reproducing various pale tones, wherein said zone has a plurality of inked areas of identical dimensions and non-inked areas of dimensions greater than or equal to those of a base cell.
The invention also provides a printed medium comprising a zone reproducing various dark tones, wherein said zone presents a plurality of non-inked areas of identical dimensions and inked areas of dimensions greater than or equal to those of a base cell.
The invention also provides a medium obtained by implementing the above-specified method.